docs: def union(self, other: 'FileLineSet') -> 'FileLineSet':
          """
          Returns a set of file lines that contains the union of the lines within
          this set and a given set.
          """
          # this isn't the most efficient implementation, but frankly, it doesn't
          # need to be.
          assert isinstance(other, FileLineSet)
          l_self = list(self)
          l_other = list(other)
          l_union = l_self + l_other
          return FileLineSet.from_list(l_union)